Polyhydroxyalkanoates (PHAS) accumulated intracellularly as an energy storage substance in microorganisms are biodegradable thermoplastic polyester and draw attention as a biodegradable plastic material. The most representative example of PHAs is [R]-3-hydroxybutyrate (3HB) homopolyester, that is, P([R]-3HB), which has similar strength to that of polypropylene and excellent biodegradability. However, these were not put to practical use as biodegradable plastic because of extremely brittle properties.
Meanwhile, biosynthesis of copolymerized polyesters such as [R]-3HB/4-hydroxybutyrate (4HB) copolymers depending on the microorganisms and carbon source used has been confirmed recently. These copolymerized polyesters exhibit a wide variety of physical properties ranging from crystalline plastic to highly elastic rubber depending on the type of constituent monomer unit and the composition of copolymer, so their use as biodegradable plastic is expected.
PHA depolymerases secreted extracellularly by microorganisms such as Alcaligenes faecalis, Comamonas acidovorans, Pseudomonas picketii, Pseudomonas lemoignei, Pseudomonas testosteroni, Penicillium pinophilum etc. have been confirmed as representative enzymes decomposing PHA. It is revealed that the active site of these enzymes is a serine residue and the enzyme activity is greatly influenced by the degree of crystallinity of polyester. Lipase produced by fungi such as Rizopus delemer etc. has also been confirmed as an enzyme decomposing PHA and is known to decompose side-chain-free PHAs such as polypropyllactone and polycaprolactone. As described, the enzymes decomposing PHA confirmed up to now are only PHA depolymerase and lipase.